Transactions with Related Parties - Summary of Base Management and Performance Incentive Fees Payable (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Total management fee incurred during the period	$ 8,832	$ 5,728	$ 16,080	$ 11,424
PNMAC Capital Management LLC [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	8,832	5,728	16,080	11,424
PNMAC Capital Management LLC [Member] | Base [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	6,839	5,728	12,948	11,424
PNMAC Capital Management LLC [Member] | Performance incentive [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	$ 1,993	$ 0	$ 3,132	$ 0